A Glo Warm thermocouple prevents gas leaks by shutting off gas flow if the heater's pilot light is not burning. The device detects the temperature of the pilot light. When it falls below a certain level, the thermocouple shuts off gas flow, preventing a leak.
Glo Warm thermocouples, like all thermocouples, work because the Seebeck effect. Named for Thomas Seebeck, this effect occurs when two wires made of different metals are joined at both ends. When one end is heated, a continuous flow of electric current circulates through the two wires. A thermocouple is a Seebeck circuit with one of the ends open. As the joined end of the thermocouple becomes hotter or cooler, it produces a voltage. The strength of this voltage is a function of the temperature which causes it. A thermocouple thermometer attaches to the open end of thermocouple and uses the amount of voltage in the thermocouple to compute a temperature.
Thermocouples have no fragile or moving parts, making them extremely reliable tools. However, thermocouples are also somewhat imprecise, as even the best thermocouples are only accurate to about one degree Celsius. For applications like preventing gas leaks, thermocouples are ideal, as they are not only very reliable, but the temperature difference between when the pilot light is burning compared to when it is not is very large.